Active sonars are used to detect the presence of submarines and other underwater objects. The detection process usually consists of an operator noting a change in the background produced by acoustic energy reflected from the object under surveillance. The receiver subsystem consists of arrays of hydrophones and preamplifiers, a spatial processor (or beam former), a set of time varying gain and automatic gain control receive channels, temporal processors/post processors and display.
Coherence, as it pertains to signal processors, means that phase as well as amplitude information is used to extract signals from a masking background. A coherent processor can be implemented in a number of ways but the most common, in present sonars, is to store a digitized replica of the transmitted pulse, to then multiply the echo bearing return by the stored replica and, finally, to integrate the product over a period equal to the transmitted pulse length. The process is then periodically repeated with a time shifted return. Such a system is known as a replica correlator (or matched filter) signal processing system. During active sonar operations any factor that prevents the signal observed at the input to the processor from being a facsimile of the stored replica will reduce the coherence and, in turn, lower the processing gain.
Those factors that can prevent full processing gain from being realized can be partitioned into three groups. The first is comprised of system related elements and takes into account the coherence loss introduced by the methodologies used to replicate the transmission, compensate for own ship motion, and preprocess the return. The second is comprised of all environment related factors, and the last considers coherence loss introduced by target size and motion. Changes in the transmitted sonar signal caused by these factors are noted as dispersive changes to the signal.
Of particular interest for this invention is the dispersion caused by the environmental factors, or the effects of the medium upon the propogation of the signal, and target dispersion which is created by the fact that a submarine or other underwater object will contain many reflecting surfaces which cause the energy of the reflected signal to be spread over a time period greater than that of the original pulse (or sonar signal).
Dispersion caused by the environment or the medium is a multi-path phenomenon and as such is dependent, to a large degree, on the number of acoustic paths that contribute to form the acoustic return. This number is, in turn, dependent on the water depth and the co-latitudinal main response axis used for transmission. The largest signal distortions (corresponding to the greatest dispersion) are obtained during shallow water operations and in deep water when bottom reflected paths are used. The effect of environmental dispersion can be modeled as a discrete number of single, separate paths between the source of the signal, the target or reflector, and the receiver.
Since the medium and target are both dispersive, the replica correlator no longer serves as a true matched filter. With dispersion, one knows intuitively that the signal that appears at the envelope detector output has a time-bandwidth product greater than 1. This suggests the possibility of interposing a post-processor between the envelope detector and the thresholder as a mechanism to regain some of the dispersive loss.